Apparent color and Raman vibrational modes of the high-temperature superconductor Bi 2 Sr 2 CaCu 2 O 8+δ exfoliated flakes

Abstract

Studying and controlling the properties of individual exfoliated materials is one of the first steps towards the fabrication of complex van der Waals systems. In this work, we present a systematic study of optical properties and micro-Raman spectroscopy on exfoliated flakes of the high-temperature superconductor Bi2Sr2CaCu2O8+δ (BSCCO-2212). We demonstrate that these are quick and non-invasive techniques for studying air sensitive materials. The apparent color of BSCCO-2212 exfoliated flakes on SiO2/Si has allowed a rough and fast identification of the number of layers. By analyzing the optical contrast of different flakes we determined the complex refractive index in the visible range and found the optimal combination of illumination wavelength and substrate properties for the identification of flakes with different thickness. In addition, we report the hardening of the most characteristic Raman modes as flake thickness decreases, possibly caused by strain in the BiO and CuO2 planes. Moreover, the evolution of the Raman modes establishes a second approach to determine the thickness of BSCCO-2212 thin flakes. As BSCCO-2212 is a challenging material due to its sensitivity to ambient conditions, the present work provides a guide for the fabrication and characterization of complex van der Waals systems paving the way for studying heterostructures based on unconventional superconductors in the 2D limit.